The present invention is directed to an inspection system for ultrasonically inspecting a material such as metal, and, more particularly, the present invention is directed to a boresonic inspection system which performs shear mode inspection of near bore material in turbine and generator rotors by passing ultrasonic search units through an axial rotor bore.
For many years, there has been increasing interest in, and a growing demand for, equipment and methods which can be used to inspect power generation turbine and generator rotors for possible material discontinuities or degradation which could lead to premature, and possibly catastrophic, failure of these components and which allow rotor life extension where appropriate. The consequences of a sudden, catastrophic failure of such a component would be severe, certainly in financial terms and possibly in terms of human looses. The center portion of the steel forgings from which these rotors are made, by the very nature of the manufacturing process, is perhaps the most suspect material in the rotor in terms of naturally occurring discontinuities and other material disorders. This is, in fact, one reason that a central bore hole is machined through most rotors in an attempt to remove this suspect material. In addition, the operating conditions at and near the central bore holes in rotors can lead to service related disorders such as thermal creep, fatigue and thermal embrittlement, especially in the presence of inherent forging discontinuities. Thus, there is a great interest in rotor inspection capabilities.
Several nondestructive test methods have been developed for use in interrogating the bore and near bore regions of rotor forgings. When the forging is new and before the final machining has taken place, it is still cylindrical or near cylindrical in shape and ultrasonic inspection from the outside has proven to be a valuable tool. However, because of the complex geometries which characterize the outer peripheries of completely machined forgings, ultrasonic inspection from the outside is impractical for inspecting rotors once they are machined. Other methods such as visual and magnetic particle examination have been used successfully to inspect the bore, but these methods are only sensitive to discontinuities which intersect or are very near to the bore and then only yield a two dimensional view of the material and any detected discontinuities.
Since the early to mid 1970's, ultrasonic inspection from the rotor bore itself has gained fairly wide acceptance as a viable volumetric inspection method. In this method, which has become known as boresonic inspection, the ultrasonic transducers are transported through the central bore hole by some convenient method and the ultrasonic beams are directed from the bore surface into the rotor material. The ultrasonic wave can penetrate well into the rotor material, and by collecting, processing, and observing any reflections of the wave which occur within the forging, one can get some idea of the integrity of the material. Volumetric inspection is achieved by scanning the transducers around the circumference and along the length of the bore while directing the ultrasonic beam into the material so that the beam has been ultimately passed through all of the material of interest.
Early borosonic test systems and some still in use, such as that described in U.S. Pat. No. 3,960,006, are based on conventional, contact ultrasonic practices. In a contact system, Plexiglas shoes (or shoes made of a similar material) are ground to the exact curvature of the bore being inspected and are mounted on the ultrasonic transducers. The search units (transducers with shoes attached) are then operated in direct contact with the bore surface, with some viscous liquid couplant medium spread over the surface to enable sound transmission from the shoe into the metal. The transducers themselves contain piezoelectric elements which generate ultrasonic waves in the compressional, or longitudinal, mode in the shoe. If a longitudinal wave is desired in the metal, the transducer element is oriented in a plane parallel or nearly parallel to a bore tangent plane; in other words, so that the incident wave in the shoe is aimed nearly radially at the bore. If an angled shear wave is desired in the metal, the plane of the search unit shoe onto which the transducer is mounted, is inclined relative to the bore tangent plane such that a refracted shear wave is produced at the shoe/metal interface. In other words, the incident wave in the shoe is not normal to the bore surface. The geometry can be such that the refracted wave travels at an angle somewhere between radial and tangential while remaining in a plane perpendicular to the bore axis (tangential aim shear), at an angle somewhere between radial and axial while remaining in a plane cut through the bore axis (axial aim shear), or a combination of the two. Typically, the refracted shear wave is on the order of 35.degree. to 70.degree.. Angled compressional wave interrogation can also be employed but this is not standard practice.
In a conventional contact ultrasonic system, the emitted wave travelling through the rotor material is divergent; that is, the wave front grows in size as it moves away from the source. The intensity of this wave therefore decreases with increasing travel distance (since the area covered by the wave is increasing) and therefore, the intensity of a wave returning from a given reflector decreases with increasing distance of the reflector from the search unit. Also, since most reflectors are small relative to the area (bem size) covered by the wave, the size of the reflector affects the intensity of the reflected wave. These principles have been long known and understood in ultrasonic testing in general and are used to provide an estimate of the size of an unknown reflector. The intensity of a reflected wave is normally converted, through the piezoelectric property of the transducer element, to a voltage which is then linearly presented as a signal amplitude on a cathode ray tube type presentation. Distance/Amplitude and Area/Amplitude relationships are determined using known reflectors in reference standards under conditions which reproduce or, at least, simulate the prevailing test conditions (bore curvature, attenuation, etc.). The total inspection system, including the search unit, transmit and receive electronics, amplifiers, displays, cables, etc., are calibrated using the known, artificial reflectors in a reference standard. Reflectors are considered to be reportable when their amplitudes exceed a specific amplitude limit which normally includes the Distance/Amplitude correction. Its size is estimated using the established Area/Amplitude relationship.
Most, if not all, of the existing boresonic test systems incorporate some sort of mechanical transport system to deliver the transducers into and along the bore. In general, these transport systems have three things in common: some mechanism, normally hydraulic or pneumatic, is used to properly position the test head within the bore; the transport system provides a means by which the transducers are held against the bore surface; and the system incorporates a scanning mechanism through which complete coverage of the bore is achieved. Other features such as automatic couplant feed have been added to some systems.
Some of the disadvantages associated with boresonic test systems using contact transducers are as follows:
1. Maintaining intimate contact between the search unit and the bore surface is a constant problem and the results of not doing so are severe and nonconservative. Any loss of contact results directly in blind areas within the rotor from which no useful data are obtained; in other words, reflectors can be inadvertently missed. Furthermore, partial loss of contact and even contact pressure fluctuations result in partial wave loss and therefore in erroneous amplitude measurements and erroneous reflector size estimates. In an attempt to overcome this deficiency, developers have instituted hard to meet requirements for extremely smooth and uniform bore surfaces. Another aid in minimizing this problem has been the development of compliant transducer transport fixtures which will follow the bore contour while maintaining contact with it.
2. Contact ultrasonic inspection in general, and as applied to boresonic inspection in particular is limited in its ability to detect reflectors lying at or near the surface from which the test is being conducted. Even if a reflector is detected, size estimates can be very inaccurate. For rotors, this is especially significant since the stresses and probability of having flaw or discontinuity indications are both highest at the bore surface and decrease with distance away from the bore. This near surface capability limitation is due primarily to two factors. First, is the effect of the shoe between the transducer and the bore surface. Part of the sound which is generated by the transducer is reflected back into the shoe at the interface of the shoe with the metal. This is a natural occurrence which results from the different acoustic properties of the two materials. This sound continues to reverberate in the shoe, with some escaping each time it strikes a boundary, until it eventually decays to an insignificant level. Until it reaches this level, however, a signal is generated each time the sound wave strikes the transducer. These signals appear to be located at or beyond the shoe/rotor interface and mask any real reflections that originate in rotor material but that are received during the time period of the unwanted reverberations. Although much has been done to eliminate or at least minimize this effect, the first 1/4 inch of rotor material is normally considered to be uninspectable. The second factor has to do with the near field characteristics of the sound beam. As a wave propagates away from the transducer, its characteristics undergo certain changes. Near the transducer, the beam is characterized by pressure maxima and minima which arise due to constructive and destructive interference of the wave front as it is forming. As the wave travels away from the transducer, the pressure fluctuations decrease, both in number and in relative magnitude until, a point is reached at which a uniform, divergent beam has been formed. This point is defined as the near field limit. The Distance/Amplitude and Area/Amplitude relationships discussed earlier can only be developed in, and are therefore only useful in, the presence of a uniform wave front. The exact point at which this occurs is a function of several variables, of which transducer size and frequency are the most significant. In general, however, the ability to detect reflectors lying near the test surface is considered to be extremely unreliable, at best.
3. Contact inspection is limited in its ability to accurately size real reflectors. Ideal reflectors, commonly flat bottom holes (with the beam normal to the flat) and side drilled holes (with the beam normal to the hole axis), are used to develop the Distance/Amplitude and Area/Amplitude relationships used for detecting and sizing reflectors with a divergent beam transducer. Since any given reflector geometry has its own reflectivity, these relationships are only valid for the type of reflector from which they were derived. Therefore, the use of a given set of relationships developed on, for example, flat bottom holes will not be accurate for other specific geometries such as spheres, off-axis discs, elliptical notches, etc. The problem is compounded further when considering irregular, randomly oriented reflectors characteristic of real reflectors in real materials. It is for this reason that size estimates for such reflectors are normally given in terms of equivalency to the ideal reflectors used to develop the calibration relationships (such as Equivalent Flat Bottom Hole Area) and do not necessarily reflect the actual size.
4. Compliant transducer support systems used to improve surface contact contribute directly to inaccuracies in estimates of the position of a reflector. Reflector position is determined from a combination of the transducer position, the beam direction, and the time of flight of the wave from the time the transducer is pulsed until the reflection arrives. A soft mechanical support system can affect the accuracy of the transducer position as well as the beam direction.
5. Resolution, as used in ultrasonic testing, is defined as the ability to discriminate between two reflectors lying in close proximity to one another. Because the ultrasonic beam in a contact system is divergent, resolution is very poor. This means, for example, that a number of relatively small reflectors could be reported as one larger reflector, an error which could affect the analysis and final disposition of the rotor.
6. Since the beam is divergent with the pressure decaying with increasing distance from the transducer, sensitivity falls off sharply with increasing depth.
More recently, a new direction regarding bore ultrasonic inspection of rotors has begun to emerge. A test system, known as TREES (Turbine Rotor Examination and Evaluation System) has been developed under the direction of the Electric Power Research Institute (EPRI) for the American Electric Power Company. This test system is the first known rotor bore inspection system to provide inspection capability based upon immersion ultrasonic testing techniques. For the purpose of this writing, TREES is categorized as a fixed focus immersion system.
Fixed focus immersion systems provide certain features which overcome many of the shortcomings of the contact systems. The transducers operate in an immersion bath which eliminates many, if not all, of the contact problems. No transducer shoes are required as the water provides a path for the sound to travel from the transducer to the rotor. The transducer can be offset from the bore by an amount which provides for the near field effects to occur entirely in the water so that the beam is formed and well behaved at the bore surface and beyond. Generation of either angled compressional or shear waves in the rotor can be easily accomplished by simply tilting the transducer such that the beam strikes the bore surface at other than normal incidence.
Focussing of the ultrasonic beam can be accomplished by fitting a lense to the transducer. The outer surface of the lense is concave and the exact geometry can be designed to achieve the desired results. For example, the lense can be spherically or cylindrically concave depending on the desired spot size geometry. The lense can also include correction for the effects of material geometry on the beam, such as correction for the focussing effect of striking a cylindrical bore surface. By carefully designing the lense geometry, in combination with a specific stand off distance from the test surface, the depth within the material at which the beam will be in focus can be controlled. Adding a focussing capability is the most significant advantage offered by using immersion ultrasonic technology. By utilizing carefully designed lenses, the ultrasonic beam can be reduced in size over some effective focal length in the material. This results in a small, high intensity beam which yields significant improvements in both sensitivity and resolution. In addition, using highly focussed beams improves the accuracy of reflector size estimates. Instead of using amplitude as compared to some ideal reflector, the reflector can be outlined by scanning the finely focussed beam across the reflector. Because the beam is small and of high intensity, reflections can be obtained from relatively poor reflecting surfaces of a reflector, thereby improving the accuracy of the size estimate. This also yields shape information which is generally unavailable from contact testing.
The primary disadvantage of a fixed focus system is that the beam only retains the high intensity, small spot size over a limited depth range. To achieve volumetric inspection with conventional immersion transducer technology, a number of specifically designed transducers must be used. In the TREES system, for example, 12 transducers (6 pairs of tangential aim transducers looking in opposite directions) are required to cover the first four inches of rotor material. When so many transducers are required, it becomes cumbersome to transport them through the bore. One must choose between retaining flexibility by including motion capabilities within the scan head for each transducer, and maintaining the capability of passing all transducers through the bore at one time and thereby performing an inspection in a single scan pass. Multiple scans are not an attractive option due to the time required to do so. Limiting the motion capabilities and crowding the transducers into as small a space as possible limits the capability of inspecting bore geometries other than a straight cylinder. Bottle bores and bottle bore transitions are very difficult to interrogate unless each transducer can be moved to the appropriate position as it passes such areas.
All of the known systems use hydraulically or pneumatically activated, articulated arm devices to move some bore riding feature, normally the shoe, to the bore surface. These have been three arm devices that take advantage of the self-adjusting nature of three contact points in circular cavity. In some of these systems the articulating center mechanism also serves as a transducer housing. There are several disadvantages associated with this type of support system:
1. The mechanical advantage of the articulating arm design, employed because the apparatus must operate in a variety of bore sizes, is variable with bore diameter. That is, when the device is in a small bore and the arms are near to their closed position, it is stiff axially and soft radially. When the same device is in a large bore and the arms are near to their full open position, it is stiff radially and soft axially. Both of these extremes can lead to problems. In a small bore situation, the radial softness can result in an inability of the centering device to fully support the weight of the scan head. In the large bore case, the radial stiffness can result in a radial deflection of the scan head from the nominal bore centerline when bore geometry irregularities such as dimples are encountered. The radial stiffness characteristics are even more important if the centering device is also being used as a transducer positioning device. It must be stiff enough, when at its lowest mechanical advantage, to support the weight of the scan head and hold the transducers in intimate contact with the bore. On the other hand, when at its greatest radial mechanical advantage, it must be soft enough to allow the arms to track the bore even in the presence of irregularities without pulling the scan head away from the bore centerline and applying a bending moment to the scan head. This is a very difficult, if not impossible, compromise to reach, especially over a wide range of bore sizes.
2. If this type of support system is hydraulically or pneumatically actuated, it cannot be adjusted to operate at a specific offset from the bore. The centering mechanism is either engaged or disengaged, depending on the status of the hydraulic of pneumatic pressure.
3. Even if motor driven, the positional accuracy would be reduced by the ratio of the radial displacement to the motor action required to achieve that displacement.
4. The position accuracy of such a device is a function of the bore diameter in which the device is being engaged since the radial motion is achieved by a specific motor action and varies with depth.
5. Uninterrupted support as the device passes steps and tapered bottle bore transitions is difficult to achieve. Typically, supports cannot be engaged in bottle bore transitions because of the irregular geometry which results from the machining done in these areas. Supports definitely cannot be engaged near steps in bores. This means that multiple supports must be included so that some may be disengaged while other are engaged and providing the necessary support. Articulated arms, if long enough to provide for fairly large bores and yet designed to fold in so that they can pass through small bores, require excessive axial space in the scan head, especially when multiple devices are provided.
In prior art contact systems, the mechanism, as mentioned above, which holds the transducers in contact with the bore surface involves one or more articulated arms. In this type of device the transducer is attached, via some pivoting feature, to one end of an arm which lies primarily in the axial direction, that is, along the rotor bore axis. The opposite end of the arm is attached, also via some pivoting feature, to some centrally located housing. An actuator is attached to the arm somewhere along its length and the transducer is moved to and held against the bore by applying a radially outward force via the actuator and allowing the arm to rotate about its attachment to the central housing. The transducer rotates about its pivot as contact is made with the bore surface until it is in complete contact with the bore. The actuator mechanism is either hydraulically or pneumatically driven in the prior art designs. In all but one known case, the articulated arms are arranged in groups of three located at the same axial position and equally spaced around the central housing. This configuration takes advantage of the self-centering capability of three point contact in a circular hole. Transducers can be mounted on any or all of the arms.
In some contact inspection systems, the tranducer locating mechanism also support the scan head and in other systems, scan head support and transducer location have been maintained as separate functions. In all cases, the transducers and scan head support mechanisms are deployed to their operating positions and then the entire assembly is driven through some scan pattern so that the transducers pass over the bore surface.
The second type boresonic system, as mentioned above, involves the application of immersion testing techniques which allow for the use of focussed ultrasonic beams and thereby offer several advantages in terms of sensitivity and resolution. In immersion testing, the transducers operate at a distance from the surface of the material being inspected and an immersion fluid is used to provide for the transmission of the sound from the source (transducer) into the metal and back when a reflection of the sound occurs. In the prior art, immersion-based boresonic inspection systems, the transducers are offset from the bore in a housing. The housing itself operates in contact with the bore and is deployed to the bore from the scan head in a manner similar to the method described for contact testing. However, only a single articulating arm is used and all of the transducers used for an inspection are mounted in a single housing. In this type of support, the transducers must be properly set within the housing so that they are at the appropriate attitudes relative to the surface when the housing is moved to the bore. Once the position of the transducer is set with the housing, it cannot be changed without removing the scan head from the bore.
Many boresonic inspection systems, as discussed above, are based upon contact transducer technology. In such systems, immersion fluid containment is not required and so a seal is not required. For immersion boresonic test systems, a variety of immersion approaches are available. First, one can provide a cavity which contains the immersion fluid around each transducer. This type of water column approach is advantageous in that the entire scan head is not submersed, but it is limited in its ability to track different diameters, tapered transistions, surface irregularities, etc. Another approach which may be taken involves immersing only the portion of the scan head which contains the ultrasonic transducers. This approach requires a seal which moves along the bore with the transducers and yet can seal on a variety of bore sizes. This option is very difficult to perfect. Another option involves immersing the entire bore and moving the circumferential and axial motion drives into the immersion bath. In the prior art immersion systems, the bore is completely immersed and the drive mechanism is designed such that the circumferential motion drive resides in the immersed portion of the scan head and the axial motion drive is outside of the bore but still in the immersion fluid. This solution has several disadvantages, including the following:
1. It is difficult and expensive to waterproof all drive components, especially electrical components such as motors, slip rings, and switches;
2. The use of an immersed circumferential drive mechanism and slip rings limits the quantity of wires which can be carried into the bore. This has an impact on the numbers and types of support devices, ultrasonic transducers, and auxiliary devices which can be placed in the scan head; and
3. The inclusion of the axial drive in the immersion fluid (but remote from the bore) requires a large immersion tank since the axial drive is fairly large and must be very stable. This requires a large volume of water which is harder to handle during various conditioning steps such as air removal, chemical treatment with wetting agents and rust inhibitors, etc.
The exact path along which the transducers move varies considerably for existing systems. One system, the TREES system mentioned previously, uses a continuous helical motion to advance the scan head down the bore as it is continuously rotated. Another manual system, uses a motorized rotation coupled with an air powered actuator for the axial advance. There are several disadvantages associated with the above-mentioned drive systems:
1. The drive system for the TREES scan head uses motors to continuously rotate the transducer assembly and an axial drive to push the entire assembly through the bore. For this type of system, the motor and position encoder for the rotation must be placed near the transducer assembly, and slip ring type connectors must be used to make the electrical connections to the transducers. The slip rings are necessary to permit continuous rotation without cable interference. There are three disadvantages to this type of drive system: A. the motor and position encoser for the rotation must be near the scan head and thus must be small enough to fit into the bore cavity; B. the cross-sectional space required for the rotational drive in the bore severely limits the number of electrical cables that can pass through to the down-bore components such as centering motors, ultrasonic transducers, etc.; and C. slip rings are expensive and often have reliability problems, especially when immersed.
2. The manual boresonic inspection system uses a drive box that is external to the bore and scan head. This drive box is constructed using limit switches and relay logic to produce alternate 400 degree rotations of the scan head. One increment of axial advance is used between each rotation. The axial advance is driven by a solenoid operated, compressed air powered linear cylinder. The solenoid is connected to the relay logic to produce an axial advance at the end of each rotation pass. The main disadvantages with this system are: A. the motions are neither readable nor controllable by the main computer; B. the incremental advance of the axial drive is fixed such that the system does not have fine positioning capabilities; C. The rotational drive system is belt controlled and does not have sufficient torque to rapidly rotate large unbalanced loads; and D. the drive rods are not held with sufficient rigidity in the drive box to permit accurate readout of the scan head position in the bore.
In the prior art, manual, pneumatic and motor driven inspection systems the control systems that move the scan head and provide position indications have been cumbersome and inaccurate due to resolver locations that require knowledge of mechanical slack in the system and positioning apparatus that does not allow for high resolution positioning. As a result, the location and size of discontinuities and flaws have been inaccurately located. Inaccurate flaw location, requires that remachining to remove flaws cover a larger area than is necessary, weakening the rotor at its highest stress area, near the bore. Inaccurate flaw location also hinders comparison of previous inspections with current inspections because it is difficult to determine whether a given flaw is a new flaws or an old flaw that has been inaccurately located due to alignment inaccuracies.